In heavy skillet or saucepan, melt butter; cook onion, stirring, until softened.
Add oregano, and salt and pepper to taste.
Add rice, stirring until thoroughly coated.
Cook over medium heat for 2 minutes, then gradually pour in stock.
Bring to boil; cover, reduce heat and simmer for 20 minutes.
Do not stir.
Stir in chicken, peas, tomatoes, walnuts, if using, and water chestnuts, if using.
Cook over medium heat until heated through.
Taste and adjust seasoning with more salt, pepper and oregano, if desired.
Transfer to platter and serve with several small bowls of condiments.